1. Field of the Invention
The object of the invention is a signal receiver and a method for controlling gain of a tuner.
2. Brief Description of the Background of the Invention Including Prior Art
A common problem during the signal reception, especially the reception of terrestrial television signals, is related to strong undesired signals close to a weak desired signal. The strong signals may be generated by a transmitter, which is closer than the transmitter of the desired signal. There are several known methods for solving this problem, but they are applicable for specific receivers only.
There is known from the U.S. Pat. No. 6,178,211 “Digital processing radio receiver with adaptive bandwidth control” a method for processing an intermediate frequency signal, in which the filter characteristic is narrowed in order to eliminate the interferences from the adjacent signals. The method can be used only in receivers, which allow controlling the width of the filter characteristic.
There is known from the European patent application No. 0903937A2 “Digital television signal receiving tuner system” a dual frequency conversion tuner, in which the first intermediate frequency filter attenuates the signals close to the received signal. However, it attenuates only signals within the passband of the first IF filter, and it does not protect against the overload of the input stages of the tuner by signals outside the passband of the IF filter, but within the passband of the input filter.
There is known from the European patent application No. 1398930A1 “Radio-frequency-signal-receiver and method of manufacturing the same” a method for optimizing the gain of a tuner, in which the input stage may be disrupted by strong signals close to a weak desired signal, where the AGC characteristic of the tuner is adjusted to compensate the effect of strong close signals. However, to determine the level of close signals, a peak power detector is used, which requires an additional filter, the filter limiting the band of peak power detection.
The present invention is designed for dual frequency conversion tuners, especially that in form of an integrated circuit chip. An exemplary structure of a tuner is shown in FIG. 1. The tuner can be implemented as a single integrated circuit chip or can be composed of several separate circuits. The tuner receives an input signal RF_IN, which is filtered by a band-pass high frequency input filter 101, which may consist of a set of wideband filters. The filtered input signal is amplified by a Variable Gain Amplifier (VGA) 102, whose gain is controlled by RF_AGC signal, generated by a demodulator. The first mixer Mixer_1 103 converts the signal to a first intermediate frequency (IF). The mixed signal is filtered by a band-pass first IF filter SAW1 104, whose passband width is narrower than the passband width of the input filter. The signal is next converted by the second mixer Mixer_2 105 to a second intermediate frequency. Next the signal is input through a Fixed Gain Amplifier (FGA) 106 to a second IF filter SAW2 107 and to a signal level detector 108. The signal from the SAW2 filter 107 is amplified by a second IF amplifier IF_Amplifier 109 with its gain controlled by IF_AGC signal, and next, the signal is output to the demodulator. The signal detector 108 sends to the demodulator the signal level of the IF signal within the passband of the first IF filter, which is used to generate the RF_AGC signal.
The described above tuner may have problems with reception of weak signals with adjacent stronger signals within the passband of the input filter. This problem has been illustrated in FIG. 2. The VGA 102 gain is controlled on the basis of the signal level measured by the detector 108, which measures the signal filtered by the SAW1 filter. The detector does not detect signals, which are outside the passband of the SAW1 filter and within the passband of the input filter but. The tuner shown in FIG. 1 uses an IF signal detector, which is much simpler and easier to build than a wideband high frequency detector. Therefore, if the tuner gains were adjusted to the level of the weak signal S1, this could lead to tuner overload by a strong signal S2. The known solution to this problem is to set such Automatic Gain Control (AGC) characteristic of the tuner that will decrease the gain of the VGA 102 amplifier, and concurrently increase the gain of the IF_Amp 109 amplifier. However, since the AGC characteristic in the known solution is constant, it requires a compromise. If the AGC characteristic would strongly limit the RF_AGC gain, it would be impossible to receive weak signals. If it would allow high RF_AGC gains, the tuner could be overloaded by strong signals close to the desired signal.